The vacuum processing of semiconductor wafers requires the loading and unloading of wafers into and from wafer processing equipment in a manner that does not cause adverse atmospheric contamination of the high vacuum environment or wafers within the processing apparatus. In addition, in order to maximize wafer throughput, it is desirable to minimize the time it takes for a typical loading or unloading sequence to be carried out. Furthermore, as wafer sizes continue to increase, as with the current trend from 150 mm and 200 mm diameter wafers to 300 mm diameter wafers, it becomes increasingly more difficult to simultaneously satisfy both contamination and throughput requirements, resulting in a compromise solution that is often far from ideal. Still further, as the wafer value increases, such as in the later stages of processing and with increasingly larger wafers that contain more devices as well as more complex devices, the exposure to financial loss due to wafer damage from an apparatus failure increases, forcing higher reliability requirements on the wafer transfer apparatus.
The majority of the prior art semiconductor wafer vacuum processing systems currently in use utilize what is referred to as a Vacuum Cassette Elevator (VCE) for wafer sizes up to 200 mm. An example of a VCE-equipped prior art wafer processing system 10 is diagrammatically illustrated in FIG. 1. The system 10 includes at least one VCE 11 which consists of a loadlock chamber 12 that can be pumped to high vacuum, an elevator assembly 13 located within the chamber 12, a front door 14 for operator access to load and unload a multiple-wafer cassette 15 when the chamber 12 is at atmospheric pressure, and a slit valve isolated interface port 16 that connects the VCE 11 to some form of wafer transfer module 17 for individually transferring wafers there through when the chamber 12 is at a high vacuum.
Typical operation of a processing apparatus 10 that is based on the use of a VCE 11 proceeds with an operator opening the door 14 of the VCE 11 and placing a new cassette 15 of wafers 18 on top of the elevator 13. The door 14 is then closed, followed by a pumping sequence which establishes an appropriate vacuum level in the VCE 11. The pumping time to arrive at a given vacuum pressure level is generally proportional to the volume of the VCE 11 and to the exposed inner surface area of the VCE 11 and of the wafers 18 contained therein. When the appropriate VCE vacuum level is reached, the isolating slit valve port 16 between the VCE 11 and transport chamber 17 is opened, permitting access to the VCE 11 by a robot arm 19 in the wafer transport module 17. The elevator 13 then positions the cassette 15 for access of the transfer arm 19 to a desired wafer 18 in the cassette 15. The robotic transfer arm 19 then extends into the VCE 11 through the slit valve port 16, captures the positioned wafer 18 and retracts back into the transport module 17 in preparation for delivery of the wafer 18 to an appropriate process module 20 of the apparatus 10. These steps are performed in reverse, with wafers being returned to a cassette 15 and the step of pumping the VCE 11 to vacuum being replaced by the step of venting the VCE 11 to atmosphere.
The prior art apparatus 10 of FIG. 1 can be used for 300 mm wafers if open wafer cassettes of comparable size are also used. However, for a number of reasons, semiconductor device manufacturers who are the end users of the wafer processing equipment prefer, and are in the process of establishing a standard for, a type of wafer carrier that is not high vacuum compatible and does not use a removable cassette 15. Such a carrier 25 is illustrated in FIG. 2. The carrier 25 includes a vertical array of horizontal wafer supporting rails 26 built into the carrier 25, typically at thirteen or twenty-five equally spaced levels. The carrier 25 has a front door 27 which is normally closed during the transport of wafers 28 between different pieces of processing equipment.
Since the carrier 25 is not high vacuum compatible and contains no cassette or cassette elevator, wafers must be transferred at atmospheric pressure from the carrier 25 and into the wafer processing equipment. The straight forward method contemplated in the prior art is to transfer wafers 28 from the carrier 25 into processing apparatus such as machine 10 of FIG. 1. Where it is desired to place a full carrier 25 of, typically thirteen or twenty-six, wafers into a large VCE 11, one would have to devise a method for quickly moving wafers from the carrier 25 into the VCE 11. A single wafer serial transfer approach adds significant time to the loading and unloading cycles, and is therefore undesirable. A multiple wafer simultaneous transfer approach has been demonstrated in which wafers are transferred in one or two batches from the carrier 25 to the VCE 11. Such a parallel transfer approach, however, presents risk of multiple wafer damage due to a single equipment failure, which is a risk preferably avoided. Also, the likelihood of mechanically touching of the back side of a wafer that is positioned over another unprocessed wafer, which is difficult to avoid when wafers are transferred simultaneously, poses potential particulate contamination problems. In addition, with a VCE dimensioned to hold wafers that are 300 mm in diameter or larger, VCE pump-down and/or vent time can be unacceptably long, making the loadlock cycle a throughput limiting factor in the operation of the processing apparatus. To compensate for these delays with a compromise of pump-down or vent time can result in an increase in atmospheric contamination of the transfer chamber or in particulate contamination on the wafer or both.
With large diameter wafers, large high vacuum pumps are necessary to pump the large VCEs that the larger wafers require. Such large pumps are difficult to mechanically isolate from the VCEs, and as a result such pumps have a tendency to transmit vibrations to the VCE that can cause particles to fall from one wafer onto another wafer below. Similarly, it is believed that the up and down motion of elevators in VCEs can also contribute to an increased vibration-induced fall of particles from upper wafers to lower wafers. Vibration can also cause wafers to move out of their positions in the cassettes and to be out of the position necessary for pick up by the transfer arms.
Accordingly, there remains a need for loading and unloading wafers into and from wafer processing equipment from a non-VCE carrier in a manner that does not cause adverse atmospheric contamination of the high vacuum environment or the particulate contamination of wafers within the processing apparatus, and that does not limit wafer throughput of the equipment, particularly with large diameter wafers, such as those of 300 mm diameter or larger, and without increasing the risk of financial loss due to multiple wafer damage from an apparatus failure which would force higher reliability requirements to be placed on the wafer transfer apparatus.